At present, there are two crystals which are distinguished in ultralviolet NLO(UV NLO) materials, one is .beta.-BaB.sub.2 O.sub.4 (C. T. Chen, B. C. Wu et al. Sci. Sin. B28,234(1985)), and the other KBe.sub.2 BO.sub.3 F.sub.2 (abbr. KBBF) (C. T. Chen, B. C. Wu et al. Nonlinear Optics: Materials, Fundamentals and applications, MA7-1/19 Aug. 17-21, 1992, Hawaii, USA), both invented and developed by Prof. C. T. Chen's research group of Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences). BBO crystal has a planar (B.sub.3 O.sub.6) group as the basic structure unit, and therefore, there is a conjugate .pi. orbital of non-symmetry in the valent orbitals of the group that is the structural reason of why the group can produce a high microscopic second-order susceptibility. Meanwhile, the groups are spatially oriented in the crystal in such a manner that makes BBO possess very high macroscopic NLO effects. In fact, the d.sub.22 coefficient, a major macroscopic NLO coefficient of BBO, is less than or equals to 2.7 pm/v, which is the highest in the ultraviolet NLO crystals found up to now. However, there are shortcomings for BBO as an UV NLO crystal. Of them, the main is the three of following:
(1) the band gap of the group is too narrow so that the absorption edge of the crystal is shifted toward the I.R. side of spectrum, reached to about 189 nm. When BBO is used to produce a harmonic generation output in range from 200 nm to 300 nm, therefore, the absorption factor is greatly enhanced, in comparison with the case when used in the visible range. It is why the crystal is so easy to be damaged when used to produce the fourth harmonic generation with high fundamental optical power. In addition, owing to the partial absorption of the quadruple frequency, the rise of temperature in the area of crystal subject to the light radiation is inhomogeneous, which leads to the local change of refractive index and greatly falling of optical quality of the harmonic generation output; PA1 (2) Due to the limit of absorption edge, as stated above, it can not used to produce a harmonic generation output shorter than 193 nm; PA1 (3) The birefringence of BBO .DELTA.n.congruent.0.12, which is also related with the planar structure of B.sub.3 O.sub.6 group arranged isolately in the crystal lattice. Such a larger birefringence of BBO makes the acceptance angle at the frequency of quadruple multiplication to be too small (.DELTA..theta.=0.45 mrad) to suit for device applications. PA1 (1) There exists a network layer structure that composes of BO.sub.3 and BeO.sub.4 group extended infinitely along the xy plane of the crystal. The atoms of BO.sub.3 group, and BeO.sub.3 atoms from the BeO.sub.4 group are arranged in a nearly co-planar manner. In this way the three oxygen terminals of the BO.sub.3 group are turned to become bridged ones to the nearest three neighbor of beryllium atom. That basically satisfies our requirements of the group design to keep a layer structure like KBBF, which therefore, make sure of large SHG effects of the crystal, and extension of the absorption edge toward about 150.about.160 nm. PA1 (2) There are oxygen atoms bridged within each pair of the layers, which belong to the out-layer coordination of the beryllium. This satisfies another important requirement of our design, which make sure of not strong layer habit of the crystal, and the better mechanical properties. PA1 (1) It overcomes, to a great extent. the strong layer habit, and appears no apparent plane of cleavage, and has better mechanical properties, in comparison with KBBF; PA1 (2) The density of the active groups BO.sub.3 in SBBO lattice is two times larger than that in KBBF, and, therefore, the SHG coefficient is about two times higher than KBBF. PA1 (3) It also overcomes shortcomings of BBO in many respects of NLO properties, such as absorption edge, birefringence, and phase-matchable range as well, while remains the SHG coefficient as same as BBO.
It was pointed out in our paper at the 1992 Hawaii meeting cited above, that it is possible to overcome the above shortcomings of BBO by replacing the active NLO group B.sub.3 O.sub.6 with BO.sub.3. It was further pointed out in the paper, that the three oxygen terminals of BO.sub.3 should simultaneously become bridged ones to other atoms, if the compound with BO.sub.3 as the basic structure unit remain the larger NLO effects of BBO, with absorption edge comparatively shifting toward the blue side of spectrum, in range 150 nm-160 nm. It is also possible for such a compound, in addition, to reduce the birefringence, which is a favor to increase acceptance angle of the crystal. Based on these theoretical considerations, we succeeded in development of a new type UV NLO crystal KBe.sub.2 BO.sub.3 F.sub.2 (KBBF), whose absorption edge reaches at 155 nm, birefringence down to about 0.7, and the phase-matchable range extends to 185 nm, So KBBF, obviously, is ideal in the three respects of above. However KBBF is found very difficult to grow, because of too strong layer habit of the crystal lattice. And relative to this, the crystal appearance is similar to mica, with a severe cleavage at (001) plane of the lattice. These make a great difficulty for KBBF is found very difficult to grow, because of too strong layer habit of the crystal lattice. And relative to this, the crystal appearance is similar to mica, with a severe cleavage at (001) plane of the lattice. These make a great difficulty for KBBF to become a practical NLO crystal.
What the aim of this invention is concerned to is to invent a novel NLO crystal, which both overcome the shortcomings of BBO and KBBF, and maintains or even exceeds the merit of BBO with respect to SHG, This innovation, in particular, helps open up a new path for development of NLO crystals for applications of vacuum UV.